1. Field of the Invention
The present invention relates to an electro-optical imaging system for converting an optical image into an electrical signal representative thereof.
2. The Prior Art
Electro-optical imaging systems are widely used in applications ranging from real-time video to character recognition in documents used in association with data processing systems.
An important class of such systems involves the use of a bucket-brigade charge-coupled image convertor. Such convertors are well known in the art and comprise one or more shift-register coupled chains of photosensitive elements where each of the elements can accumulate electrical charge representative of the light incident thereon and where the electrical charge so accumulated can be passed down the chain or chains, from element to element to an output port. The image to be electro-optically converted is focused onto the chain or chains of elements, a time is allowed for the electrical charge to accumulate, and then the charges are moved, as output, to the port.
Those areas which receive little light during the charge accumulation period accumulate little electrical charge, and those areas which receive much light accumulate larger electrical charges. The movement of the charges down a chain does not inhibit the charge accumulation process. Thus, a charge packet from an element in receipt of a dim portion of the image can pass through an element which is in receipt of a bright portion. The amount of charge in the package is added to, the amount of addition being greater the further from the port that the charge originating element lies. This process degrades the sharpness of edges in the image the effect being known as "smearing" and further limits the dynamic range of the charge-coupled device by placing a lower limit on the darkness representative signal.
In order to reduce the spurious charge accumulation effect various modifications have been applied to the design of the charge-coupled device. In a first scheme each chain of photo-sensitive, charge accumulating elements has provided, in parallel therewith, a shift register chain which is shielded from the light. The accumulated charges are first shifted in parallel into the light shielded chain and thereafter moved, serially down the light shielded chain, to the port. In a second scheme intermediate light-immune storage is provided on the charge coupled device. The charges from the photo-sensitive chain are shifted very rapidly into the intermediate storage so that the amount of time of exposure to extraneous charge-accumulating light in other elements is minimized. Thereafter the charges, or representations thereof, can be presented to the output port at a desired rate of presentation by their retrieval from the intermediate storage. While both of these schemes are successful in eliminating or reducing the accumulation of unwanted charge, they both have the disadvantage of requiring the addition of further elements onto a photosensitive device whose cost is already high and where the areal density of photosensitive elements is generally required to be increased rather than decreased.
The fabrication of such charge coupled devices has, in common with the fabrication of all other semiconductor devices, an individual device fabrication failure rate which rises steeply with the area of the device. The elimination of such extra structures on a charge-coupled photo-imaging convertor would reduce the area of the device. This real reduction would be of particular use in those devices where plural rows of photo-receptive elements are serially accessed to provide representation of whole areas of images. The increased yield of these large-area devices would considerably reduce their cost.
In one area of use of electro-optical imaging systems, namely the machine reading of documents, it is usual to employ a single-chain charge-coupled image convertor, the image of the document to be read being focused onto the single chain and the document swept across the field of view so that the image of the document is obtained as a series of strips akin to a television picture. While the single chain image converter is of a relatively low cost, it ideally still requires the addition of spurious charge accumulation preventative measures as described, and its use requires the inclusion of mechanical components for moving the document or its image. Such components presenting cost, precision and reliability problems.
It is a problem that the number of photosensors per unit area in charge-coupled, bucket-brigade electro-optical image convertors is sometimes not sufficient for adequate resolution of the components in an image. The image can be focused, at increased size, onto more than one device, but this can prove costly and the resulting, fainter incident image can cause signal to noise ratio problems.
Accordingly, it is desirable to provide an electro-optical image converting system wherein the accumulation of spurious charge, as described, is preventable without recourse to additional structures on the device itself. It is further desirable to provide such an image convertor which is free of mechanical moving parts. It is yet further desirable to provide such an image convertor where the resolution can be improved over that obtainable using a single charge coupled device without recourse to the use of more than one charge coupled device.